Passive infrared detectors are commonly found in homes, commerical offices, manufacturing facilities, and other locations where the security of persons and property may be compromised by an unauthorized person. Such systems are responsive to the presence of infrared energy radiated by the body of the intruder and are operative to provide an alarm indication signaling the detection of the infrared energy. Typically, one or more detector assembly is positioned high about the walls or on the ceiling of the region to be secured against intruders so that the field of view of each of the instruments monitors a region for the presence of infrared energy in unauthorized places and at unauthorized times.